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Title: Estimation of summary protective efficacy using a frailty mixture model for recurrent event time data
Authors: Xu, Y. 
Cheung, Y.B. 
Lam, K.F.
Milligan, P.
Keywords: Event dependence
Expectation-maximization (EM) algorithm
Frailty mixture model
Louis's formula
Summary protective efficacy
Issue Date: 20-Dec-2012
Citation: Xu, Y., Cheung, Y.B., Lam, K.F., Milligan, P. (2012-12-20). Estimation of summary protective efficacy using a frailty mixture model for recurrent event time data. Statistics in Medicine 31 (29) : 4023-4039. ScholarBank@NUS Repository.
Abstract: Recurrent event time data are common in experimental and observational studies. The analytic strategy needs to consider three issues: within-subject event dependence, between-subject heterogeneity in event rates, and the possibility of a nonsusceptible fraction. Motivated by the need to estimate the summary protective efficacy from recurrent event time data as seen in many infectious disease clinical trials, we propose a two-part frailty mixture model that simultaneously accommodates all the three issues. In terms of vaccine action models, the proposed model is a combination of the 'all-or-none' and the 'leaky' models, and the summary protective efficacy is a unified measure of the vaccine's twofold effects in completely or partially protecting the vaccinated individuals against the study event. The model parameters of interest are estimated using the expectation-maximization algorithm with their respective variances estimated using Louis's formula for the expectation-maximization algorithm. The summary protective efficacy is estimated by a composite estimand with its variance estimated using the delta method. The performance of the proposed estimation approach is investigated by a simulation study. Data from a trial of malaria prophylaxis conducted in Ghana are reanalyzed. © 2012 John Wiley & Sons, Ltd.
Source Title: Statistics in Medicine
ISSN: 02776715
DOI: 10.1002/sim.5458
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